North equatorial countercurrent in the tropical Atlantic: Multi- jet structure and seasonal variability

Abstract

The goal of this paper is to study the typical seasonal cycle of the North Equatorial Countercurrent (NECC) in the whole Tropical Atlantic Ocean using hydrographic and CTD data of 1911 to 1992. Profiles of density in the upper 500 m layer were calculated using monthly profiles of temperature and salinity averaged over one degree mesh and standard state equation. Then the typical seasonal cycle of density (monthly av eraged over entire period of observation) was analysed using the Fourier method. The sum of the averaged magnitude and annual/semi-annual harmonics characterizes a “pure” seasonal cycle. Monthly zonal geostrophic currents were calculated using “pure” monthly density fields in the northern Tropical Atlantic Ocean from 3° to 24° N. The level of no motion was postulated at 500 m. Zonal currents were obtained at the me ridional sections along 20δ, 23δ 30δ, and 45δ W. Then, a zonal transport was calculated in the following lay ers: 0 to 50, 0 to 100, 0 to 200 and 0 to 500 m. Besides, monthly meridional geostrophic currents were calculated along 3°, 5° and 8° N using the same algorithm. Our results confirm the existence of two NECC jets in the western and central Tropical Atlantic Ocean (from April to September and from September to De cember, respectively). At the same time, they show that there are three jets of the NECC during the first half of the year. The maximum total transport of the NECC in the central Tropical Atlantic Ocean is about 60 Sv (in November). Three branches of the NECC are also observed during the typical seasonal cycle in the vi cinity of 20° W. The multijet structure of the NECC in the whole Tropical Atlantic Ocean and the phases of the annual and semi-annual harmonics confirm that there are several sources generating the NECC. The possible generators are discussed in detail. Our results confirm that there are three main sources of the NECC, namely Ekman pumping, Rossby waves, and meridional monsoon winds in the eastern Tropical At lantic Ocean. The bottom relief and West Africain shore configuration are responsible (at least partly) for the NECC bifurcation in the eastern Tropical Atlantic.